How can I tell the sex of my budgerigar?

The simplest way is by checking the cere which is the fleshy part above the beak. A male budgie will almost always have a dark blue cere. If it's a very young bird, the cere will be purple.

Females usually begin with a very light blue cere and as they age, their cere will turn to a darker light blue, but not as dark as a male.